Advances in magnetic storage technology require that data rates and especially data write times be reduced. This means that a typical magnetic storage device employing a read-head to read data from and write data to a magnetic medium, e.g., a disk or a tape, has to improve the efficiency of the head. In particular, the rate at which data is written has to be increased, since write and read data rates are the same.
The head is usually formed from two or more elongated pieces of a suitable ferromagnetic material such as a nickel iron (NiFe) alloy. The poles are joined at one end, called the yoke, and separated by a precisely defined gap at the opposite end called the tip. A coil is wrapped around the poles near the yoke. During operation the head is positioned with the tip adjacent the magnetic medium. Data is written to the magnetic medium by driving magnetic flux into the head by passing a write current through the coil. When reading data, the coil senses magnetic flux from the head by generating a read current. The electromagnetic parameters governing the behavior of such heads dictate that the yoke be wide to increase magnetic flux passing through the coil and that the tip be narrow to accommodate high data track density in the magnetic medium.
At high track densities the magnetic properties of the materials used in the poles become crucial. Specifically, what is required is a pole material exhibiting a high magnetic permeability .mu. and low magnetic coercivity H.sub.c. Since ##EQU1##
where M.sub.S is the magnetization at saturation and H.sub.k is the anisotropy magnetic field, a high .mu. suggests a choice of H.sub.k as small as possible. However, it has been shown by Nakamura, et al., IEEE Trans. Mag. 21(5), 1985 that too low an anisotropy results in undesirable magnetic domain patterns in narrow pole tips. However, increasing H.sub.k to a high value has the adverse effect of decreasing the reproducing sensitivity because of reduced .mu..
The prior art suggests that the above problem be solved by using laminated poles which combine beneficial magnetic properties of two materials. For example, in U.S. Pat. No. 3,639,699 Tiemann discloses a thin film structure of permalloy with a high .mu. and a second magnetic material with low .mu. and high MS. Lazzari teaches in U.S. Pat. No. 3,867,368 to use an additional decoupling layer between laminations of high .mu. and low .mu. materials to eliminate exchange coupling between them. Lazzari et al. further explain in "Integrated Magnetic Recording Heads", IEEE Trans. Mag., Vol. MAG-7, No. 1, March 1971 that obtaining single domain behavior in laminated poles is desirable for efficient operation of the read-head.
The problems associated with the use of laminated poles include domain stability, material composition, reduction of eddy currents, proper dimensioning and parametrization of the magnetic properties of the layers (e.g., determination of the easy magnetization or fast axis and minimization of edge domains). These general problems are addressed by Nakanishi in U.S. Pat. No. 5,018,038; Andricacos et al. in U.S. Pat. No. 5,132,859; Re et al. in U.S. Pat. No. 5,142,426; Shukovsky et al. in U.S. Pat. No. 5,157,570; Campbell et al. in U.S. Pat. No. 5,264,981; Jeffers et al. In U.S. Pat. No. 5,239,435; Ohkubo et al. in U.S. Pat. No. 5,313,356 and Arimoto et al. in U.S. Pat. No. 5,576,098. Additional patents addressing laminated poles include U.S. Pat. No. 4,610,935; U.S. Pat No. 5,108,837; U.S. Pat. No. 5,224,002; U.S. Pat. No. 5,576,099 and U.S. Pat. No. 5,606,478.
Although the teachings of the above references do solve many intervening problems and make laminated poles more efficient, they fall short of presenting a head capable of high speed writing. That is because in most of the yoke the magnetic flux runs substantially parallel to the magnetic lamina, but in the tip, specifically at the gap, the flux is generally perpendicular to the lamina. As a result, there is a magnetomotive force drop across the accumulation of non-magnetic layers which reduces the efficiency of the head. Even when using high .mu. materials (relative .mu.=1,000) the effective average permeability perpendicular to the lamina in which a fraction f of total thickness is made of magnetic material will be at maximum at about 1/(1-f). As a result, excessively high write currents are required for such heads.
U.S. Pat. No. 5,590,008 to M. Tanube et al. teaches that the upper pole may include two or more layers and a metal may be placed in the gap between the poles. This arrangement will aid in reducing the write current to some extent. In addition, Harry Gill in "CoHfNb/Al.sub.2 O.sub.3 Laminated Write Pole for an Integrated Spin Valve Giant Magnetoresistive Read Inductive Head", IBM Technical Disclosure Bulletin, Vol. 40, No. 4, April 1997 teaches to improve the write efficiency at high frequencies by using a high electrical resistivity, non-laminated write pole made of a Co-based amorphous alloy to further reduce eddy currents.
Unfortunately, a problem not solved by the addition of a non-laminated pole portion at the tip or placement of metal in the gap between poles is the alignment of domain magnetizations at the narrow end of the pole in the tip. Specifically, for efficient operation the domains in the tip should have a tendency to align along the easy or fast axis of the high .mu. layers. However, edge effects at high frequencies prevent this from happening. Additionally, all prior art laminated heads have at least one non-planar pole for accommodating the coil. This geometry negatively affects the magnetic properties of the head because it interferes with efficient conversion of current to magnetic flux in high .mu. layers and vice versa.